I haven't ridden the SL or any NS board for that matter but I've had the same issues as you with the rocker-camber boards I have ridden. I enjoy the low speed playfulness and the pop of the cambered tips but I never feel comfortable flat basing at high speeds. I think I'm too light (160lb.) to keep the board pressed into the snow and that nose always wants to lift and it feels like it's going to catch when it comes back down. Next RC board I try I will be downsizing from my usual 155-157 to a 152-154 to see if bringing the camber nearer my feet affords me more control. I'm also going to be looking for something with a little softer flex so that I'll hopefully be able to manipulate the board better with my limited poundage.
Just got my first flat camber board, a nitro rook, so I'm exited to try that profile out to see if it's more stable when flat basing.